1. Field of the Invention
The present invention relates to paper machines, and, more particularly, to rolls used in press assemblies of paper machines.
2. Description of the Related Art
The application of suction rolls, especially press suction rolls in press sections of paper machines limits the maximum line force of presses, because the shell is weakened by the drilled holes through the shell. A great number of holes is needed to get a great open area, which gives a good, safe and sufficient dewatering behavior.
During development of an air press process, it has been found that once air passes through the sheet, there are inadequate flow channels to allow the air to pass out of the system. Grooves or blind drilled holes in the roll impede the flow of air out of the system. The reason for this is simple. The exit path for the air requires that the air pass into the grooves and then flows within the grooves until it exits under the cap-main roll nip. Because of the geometry of the grooves and because of the need to prevent breakage of the land areas between the grooves, there can never be enough capacity in a grooved roll, no matter what size groove is used.
Due to the quantity of air needed for dewatering, it has been found that air velocity in the grooves may become supersonic in order to handle all the air needed for the process. The need for supersonic air flow really means that the grooves cannot handle the flow, and that they will present a tremendous backpressure to impede dewatering.
In addition to the need for a low impedance path for airflow, there are also other imposed constraints. One concern has to do with the huge mechanical load impressed on the main roll. In a known air press, the pressure arc can be 1 meter long in the machine direction. This arc length times the width of the machine gives the total pressurized surface area in the pressure chamber. This surface area multiplied by the air pressure equals the total load impressed on the main roll by the air pressure. This air load can be huge, and it has been found that substantial rolls will be needed to support the load. This puts severe limits on the roll construction since rotating shell constructions similar to suction rolls cannot be easily used.
Another constraint for an air press relates to the support of the fabrics as they pass through the pressing process. It has been found that the main roll needs to be reasonably flat. If there are wide valleys or groove in the main roll, the dewatering fabrics will dip into these valleys, and this can cause a leak in the main roll to cap roll nip seal. Air can escape under the cap roll in channels along the grooves if the grooves are too wide.
Yet another constraint is the need for the uniform flow of air through the sheet. As groove spacing increases, airflow becomes less uniform. As uniformity decreases, more air is needed to dewater the sheet.